The term “induction heating” generally describes a process in which an alternating current is passed through a coil to generate an alternating magnetic flux. When the coil is placed in close proximity to or wrapped around a metallic object that is to be heated, the alternating magnetic flux inductively couples the load to the coil and generates eddy currents within the metallic object causing it to become heated. Because of its function, the coil is often referred to as a “work coil” or “induction head,” and the metallic object to be heated as a “load.” Induction heating may be used for many purposes including curing adhesives, hardening of metals, brazing, soldering, welding, and other fabrication processes in which heat is a necessary agent or catalyst.
The field of induction heating is considered to be well-established, with several types of induction heating systems having been developed to control power delivered to the induction head and, thus, the heat produced in the load. One type of induction heating system, sometimes referred to as a resonant system, generally comprises a power supply, a resonant induction head typically formed by the work coil and a capacitor, and some type of switching means to control delivery of power to the resonant induction head by the power supply. Generally, the switching means is closed to cause the power supply to provide a current to the resonant induction head resulting in energy being stored in the work coil. When the switching means is opened, the induction head begins to resonant and generate an oscillating voltage and corresponding oscillating current, and the stored energy is discharged to the load as heat.
The greatest amount of energy is transferred from the induction head to the load during the first half-cycle of oscillation. Thus, to provide the quickest and most efficient heating of loads, conventional induction heating systems are often configured to replenish the stored energy to the induction head by operating the switching means when the oscillating voltage reaches zero at the end of the first half-cycle. However, this often does not coincide with a zero voltage at the switching means resulting in potential stress to the switching means, or requires complicated switching means to do so.
Induction heating systems, particularly those employing resonant induction heads, would benefit from a simplified scheme that substantially minimizes stress to the switching means while still providing quick and efficient load heating.